Some vehicles, such as unmanned aerial vehicles (UAVs), use a global positioning system (GPS) for navigation. GPS navigation systems receive signals from satellites and estimate the position of the vehicle based on these signals. However, the position estimation may suffer from inaccuracies sometimes inherent in less sophisticated GPS navigation systems due to, for example, noise or other problems receiving the signals. In addition, GPS navigation systems may not be able to receive the signals when, for example, the vehicle is in a location where the signals are blocked by structures, such as buildings, bridges, or tunnels, or the signals are corrupted by interference. As a result, such GPS navigation systems may not be sufficiently accurate or reliable for some implementations.